A gas turbine engine generally includes one or more compressors followed in the flow direction by a combustor and high and low pressure turbines. These engine components are arranged in serial flow communication and disposed about a longitudinal axis centerline of the engine within an annular outer casing. The compressors are driven by the respective turbines and compressor air during operation. The compressor air is mixed with fuel and ignited in the combustor for generating hot combustion gases. The combustion gases flow through the high and low pressure turbines, which extract the energy generated by the hot combustion gases for driving the compressors, and for producing auxiliary output power.
Various types of turbofan engines contain a booster section disposed upstream of the compressors. The booster section typically includes a large, annular cavity. The engine power is transferred either as shaft power or thrust for powering an aircraft in flight. For example, in other rotatable loads, such as a fan rotor in a by-pass turbofan engine, or propellers in a gas turbine propeller engine, power is extracted from the high and low pressure turbines for driving the respective fan rotor and the propellers.
It is well understood that individual components of turbofan engines, in operation, require different power parameters. For example, the fan rotational speed is limited to a degree by the tip velocity and, since the fan diameter is very large, rotational speed must be very low. The core compressor, on the other hand, because of its much smaller tip diameter, can be driven at a higher rotational speed. Therefore, separate high pressure and low pressure turbines with independent power transmitting devices are necessary for the fan and core compressor in aircraft gas turbine engines. Furthermore since a turbine is most efficient at higher rotational speeds, the lower speed turbine driving the fan requires additional stages to extract the necessary power.
Many new aircraft systems are designed to accommodate electrical loads that are greater than those on current aircraft systems. The electrical system specifications of commercial airliner designs currently being developed may demand up to twice the electrical power of current commercial airliners. This increased electrical power demand must be derived from mechanical power extracted from the engines that power the aircraft. When operating an aircraft engine at relatively low power levels, e.g., while idly descending from altitude, extracting this additional electrical power from the engine mechanical power may reduce the ability to operate the engine properly.
Traditionally, electrical power is extracted from the high-pressure (HP) engine spool in a gas turbine engine. The relatively high operating speed of the HP engine spool makes it an ideal source of mechanical power to drive the electrical generators connected to the engine. However, it is desirable to draw power from additional sources within the engine, rather than to rely solely on the HP engine spool to drive the electrical generators. The low-pressure (LP) engine spool provides an alternate source of power transfer, however, the relatively lower speed of the LP engine spool typically requires the use of a gearbox, as slow-speed electrical generators are often larger than similarly rated electrical generators operating at higher speeds. The boost cavity of gas turbine engines has available space that is capable of housing an inside out electric generator, however, the boost section rotates at the speed of the LP engine spool.
Also, it is difficult to allocate additional space inside the gas turbine engine in which to place components such as generators, because most of the available space inside the nacelle is utilized.
Use of machines operable as either generators or motors for shaft power transfer in gas turbine engines is known in the art. Hield et al. in their U.S. Pat. No. 5,694,765 which issued Dec. 9, 1997, describe a multi-spool gas turbine engine for an aircraft application, which includes a transmission system operated to transfer power between relatively rotatable engine spools. In a number of embodiments, each shaft is associated with a flow displacement machine operable as a pump or a motor, and in other embodiments, permanent magnet or electromagnetic induction type machines operable as motors or generators, are drivingly connected via an auxiliary gearbox to a flow-driven gearbox. However, Hield et al. shaft power transfer system does not disclose differential geared gas turbine engines, because they direct themselves to the transfer of shaft power between two independently rotatable (i.e. not differentially-geared) engine spools.
Rago et al., in their U.S. Pat. No. 6,895,741, which issued May 24, 2005, describe a differentially-geared gas turbine engine with motor/generator regulating mechanisms. Rotatable loads are driven by differential gearing operatively coupled with the turbine, and power transfer is controlled with machines operable as a generator or motor for selectively taking power from one of the rotatable loads to drive the other of the rotatable loads. The differential gearing system comprises a sun gear affixed to the forward end of the turbine rotating shaft, and planet gearing engaging the sun gear operatively connected to the compressor for rotationally driving the compressor at a first output rotational speed with respect to the turbine. A planet carrier is provided for operatively supporting the planet gearing and is rotatable together with the planet gearing. The planet carrier is operatively connected to the rotatable load for driving the rotatable load in a rotational motion at a second output rotational speed with respect to the turbine. The first and second motor/generator mechanisms are preferably permanent magnet motor/generators.
Therefore, there is a need for an electrical generator integrated within the boost cavity of a gas turbine engine with a high rotational speed and that does not obstruct airflow within the engine.